The present invention relates broadly to an error correction apparatus and in particular to a feature referenced error correction apparatus.
In the prior art, there has been a continuing effort to develop radar systems which are suitable for high resolution application such as ground-mapping and air reconnaissance. Initially, this finer resolution was achieved by the application of pulse-compression techniques to conventional radar systems which were designed to achieve range resolution by the radiation of a short pulse, and angular, or azimuth, resolution by the radiation of a narrow beam. The pulse-compression techniques provided significant improvement in the range-resolution of the conventional radar systems, but fine angular resolution by the radiation of a narrow beam still required a large diameter antenna which was impractical to transport with any significant degree of mobility. Subsequent to the development of pulse compression techniques, synthetic aperture radar techniques were developed for improving the angular resolution of a radar system to a value significantly finer than that directly achievable with a radiated beamwidth from a conventional antenna of comparable diameter.
In general, synthetic array processing requires that a quadratically varying phase correction i.e., an azimuth focus reference function be applied to the pre-summed sequentially derived signal vectors before integrating them, with (or without) amplitude weighting, to obtain each resolved element's amplitude. This quadratic azimuth focus reference function is calculated as a function of range, velocity, look angle and wavelength. Multiplication of successively derived signal vector returns from a particular ground scatter by a quadratic focus reference function, and integration of the resulting products is in essence the correlation process. Many parameters may affect the accuracy of this quadratic phase correction such as a change in direction of the aircraft deriving the data, variations in the height of ground features, and errors in the velocity data which, even when derived from inertial platforms, may be only marginally accurate to achieve optimal focus.